Oil country tubular goods such as tubing and casing used in the excavation of oil wells for recovery of crude oil or gas oil are normally connected with each other using tubular threaded joints. In the past, the depth of oil wells was 2,000-3,000 meters, but in deep wells such as recent offshore oil fields, the depth can reach 8,000-10,000 meters. The length of an oil country tubular good is typically 10 some meters, and tubing having a fluid such as crude oil flowing in its interior is surrounded by a plurality of casings, and hence the number of oil country tubular goods which are connected together can reach a huge number of a thousand or more.
In their environment of use, tubular threaded joints for oil country tubular goods are subjected to loads in the form of tensile forces in the axial direction caused by the weight of oil country tubular goods and the joints themselves, complex pressures such as inner and outer pressures, and geothermal heat. Therefore, they must be able to guarantee gas tightness without being damaged even under such severe environments.
A typical tubular threaded joint used for connecting oil country tubular goods has a pin-box structure constituted by a member which has male (external) threads and is referred to as a pin and a member which has female (internal) threads and is referred to as a box. Typically, a pin is formed on both ends of an oil country tubular good, and a box is formed on the inner surface of both sides of a threaded joint component (a coupling).
As shown in FIG. 1, a threaded joint which has excellent gas tightness and is referred to as a special threaded joint has a seal portion and a shoulder portion (also referred to as a torque shoulder) on each of the pin and the box. The seal portion is formed on the outer periphery near the end surface closer to the end of the pin than the male threads and on the inner periphery on the base of the female threads of the box, and the shoulder portion is formed on the end surface at the end of the pin and on the corresponding rearmost portion of the box. The seal portion and the shoulder portion constitute an unthreaded metal contact portion of the pin or box of the tubular threaded joint, and the unthreaded metal contact portion and the threaded portion (male or female threads) constitute a contact surface the pin or box thereof. By inserting one end (a pin) of an oil country tubular good into a coupling (a box) and tightening the male threads of the pin and the female threads of the coupling until the shoulder portions of the pin and the box are made to abut and then interfere with a suitable torque, the seal portions of the pin and the box intimately contact each other and form a metal-to-metal seal, thereby maintaining the gas tightness of the threaded joint.
When tubing or casing is being lowered into an oil well, due to various problems, a threaded joint which was once tightened is sometimes loosened, the threaded joints are lifted out of the oil well, then they are retightened and lowered into the well. API (American Petroleum Institute) requires galling resistance so that gas tightness is maintained without the occurrence of unrepairable seizing referred to as galling even when a joint undergoes tightening (makeup) and loosening (breakout) 10 times for a joint for tubing and 3 times for a joint for casing.
In order to increase galling resistance and gas tightness when performing makeup of a threaded joint for oil country tubular goods, a viscous liquid lubricant (a lubricating grease) which is referred to as compound grease and which contains heavy metal powders is applied to a contact surface of a threaded joint (namely, to the threads and the unthreaded metal contact portion of the pin or box). Compound grease is prescribed by API Bulletin 5A2.
In the past, it has been proposed to subject a contact surface of a threaded joint to various types of surface treatment such as nitriding, various types of plating including zinc plating and composite plating, and phosphate chemical conversion treatment to form one or more layers in order to increase the retention of compound grease or improve sliding properties. However, as described below, the use of compound grease poses the threat of adverse effects on the environment and humans.
Compound grease contains a large amount of heavy metal powders such as zinc, lead, and copper powders. At the time of makeup of a threaded joint, grease to which has been applied is washed off or overflows to the exterior surface, and there is a possibility of its producing adverse effects on the environment and particularly on marine life due to harmful heavy metals such as lead. In addition, the process of applying compound grease worsens the work environment and working efficiency, and there is a concern of its toxicity towards humans.
In recent years, as a result of the enactment in 1998 of the OSPAR Convention (Oslo-Paris Convention) aimed at preventing marine pollution in the Northeast Atlantic, strict environmental restrictions are being enacted on a global scale, and in some regions, the use of compound grease is already being regulated. Accordingly, in order to avoid harmful effects on the environment and humans during the excavation of gas wells and oil wells, a demand has developed for threaded joints which can exhibit excellent galling resistance without using compound grease.
As a threaded joint which can be used for connecting oil country tubular goods without application of compound grease, the present applicants proposed in WO 2006/104251 a tubular threaded joint in which the contact surface of at least one of a pin and a box is coated with a two-layer coating having a viscous liquid or semisolid lubricating coating and a dry solid coating formed atop it. The dry solid coating can be formed from a thermosetting resin such as an acrylic resin or from an ultraviolet curing resin. The viscous liquid or semisolid lubricating coating has tackiness so that foreign matter easily adheres thereto, but by forming a dry solid coating atop it, the tackiness is eliminated. Since the dry solid coating is destroyed at the time of makeup of a threaded joint, it does not interfere with the lubricating properties of the lubricating coating disposed beneath it.
In WO 2007/42231, the present applicants disclosed a threaded joint having a thin lubricating coating without tackiness which contains solid lubricant particles dispersed in a solid matrix exhibiting plastic or viscoplastic rheological behavior (flow properties) on the threads (of a pin and a box). The matrix preferably has a melting point in the range of 80-320° C., and it is formed by spray coating in a molten state (hot melt spraying), by flame coating of powder, or by spray coating of an aqueous emulsion. A composition used in the hot melt method contains, for example, polyethylene as a thermoplastic polymer, wax (such as carnauba wax) and a metal soap (such as zinc stearate) as a lubricating component, and calcium sulfonate as a corrosion inhibitor.
In WO 2006/75774, the present applicants described a tubular threaded joint in which the contact surface of at least one of a pin and a box is coated with a two-layer coating comprising a solid lubricating coating comprising a lubricating powder and a binder, and a solid corrosion-preventing coating which does not contain solid particles formed atop the solid lubricating coating.    Patent Document 1: WO 2006/104251    Patent Document 2: WO 2007/42231    Patent Document 3: WO 2006/75774